Dear Simon,
in a concerted calibration effort suppported by IUPAC, laboratories from
USGS Reston, IAEA, ETH Zurich, CIO Groningen, UFZ Leipzig, ANU Canberra
and MPI Jena have analyzed a number of reference materials for firmly
establishing their d18O (VSMOW) values. The substances included the
sulfates NBS-127 and IAEA-SO-5 and SO-6, the benzoic acids IAEA-601 and
602, the caffeine IAEA-600, the nitrates USGS34 and 35 plus IAEA-NO-3,
and a set of reference water samples (VSMOW, Slap, Gisp and a heavy
water at +80 per mill). We considered IAEA-CH3 cellulose but discarded
it for use as a dependable international reference material due to
hygroscopicity issues. This does not exclude cellulose as (temporary,
well controlled) local reference material, in fact, this is needed when
cellulose is routinely analyzed.
The major culprit has been the direct analysis of the water samples in
silver cups. There are multiple systematic sources of error which will
be discussed in the upcoming report. The take home message was
formulated by Harro Meijer: "Do not try this at home".
A number of further difficulties arose when comparing results from the
different laboratories, some related to the presence of nitrogen eluting
before the CO peak, some related to the peak shape (sulfates). In
general, it was learned that great care must be taken and that results
from a single laboratory, even if very precise, often do not compare
well with those from another lab, although comparably precise. The
systematic, mainly chemical effects need to be studied further in order
to find and commonly accept conditions for the high temperature reaction
that are both robust and accurate.
The results from the more than 10000 analyzes made are still subject to
a final revision before publication, but they will not change by much:
Material type / name; d18O; 1s; err(mean); ?old? values; change
_______________________________________________________
IAEA-601 benzoic acid 22.97 0.18 0.02 23.2 -0.23
IAEA-602 benzoic acid 71.23 0.42 0.07 71.4 -0.17
IAEA-600 caffeine -3.66 0.47 0.11
USGS35 56.87 0.36 0.08 57.5 -0.63
USGS34 -27.83 0.36 0.08 -27.93 +0.10
IAEA-NO-3 25.29 0.27 0.06 25.61 -0.32
IAEA-SO-5 12.14 0.20 0.05 11.99 +0.15
IAEA-SO-6 -11.19 0.20 0.04 -11.34 -0.15
NBS127 8.51 0.21 0.04 8.59 -0.08 (9.3 -0.79)
_______________________________________________________
Notes: 1s is the standard deviation across the different laboartories,
the error of the mean gives the final uncertainty taking the statistical
errors into account.
The value for the caffeine should be taken with some caution. The
reaction seemed to be rather different in the different types of
reactors with some labs seeing only a small N2 peak preceding CO,
whereas others found almost complete yield for N2. The isotopic results
varied by a large margin correspondingly. The average value in the table
is given after removing suspected outliers.
For NBS127, two values meander through the literature; the +9.3 per mill
value seems to be seriously in error (by -0.79 per mill).
Regards Willi
Simon Poulson wrote:
> Dear All,
>
> We were planning to run some cellulose-d18O analyses in our lab, and I
> had a question about d18O standards. A number of papers that have run
> similar analyses state that they calibrate a working cellulose
> standard (e.g. from Aldrich or Sigma) for d18O and use this when
> running samples, but the papers don't say what standard they use to
> calibrate this working standard.
>
> Cellulose standard IAEA-CH-3 is available from IAEA, but is certified
> for d13C only, not d18O. Is there a consensus d18O value that people
> use for this standard? Or do people use other certified standards,
> such as benzoic acid IAEA-601 (+23.3 per mil), or barium sulfate
> NBS-127? (+9.3 per mil)
>
> Thanks in advance for any comments and suggestions.
>
> Cheers,
> Simon Poulson
>
>
--
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Willi A. Brand, Stable Isotope Laboratory [log in to unmask]
Max-Planck-Institute for Biogeochemistry (Beutenberg Campus)
Hans-Knoell-Str. 10, 07745 Jena, Germany Tel: +49-3641-576400
P.O.Box 100164, 07701 Jena, Germany Fax: +49-3641-577400
http://www.bgc-jena.mpg.de/
http://www.bgc-jena.mpg.de/service/iso_gas_lab/
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